The Pentek 151122 O-ring is also known as the Pentek OR-100 O-ring. The Pentek 151122 OR-100 o-ring can be found here:
http://www.waterfilters.net/Pentek-C..._p_11-189.html
The integrity of the gasket is very important. If you don't replace your gasket enough your system will develop a leak. There is no set time period for the life of an o-ring. Replacing the gasket every time or every other time you replace your filters will ensure that your gaskets are in tact.